This invention relates to threaded fasteners. More particularly, this invention relates to a device for and method of lubricating threaded fasteners and other threaded connections.
Bolts are roughly cylindrical connector elements having external threads whereas nuts are annular in shape and have internal threads that mate with the external threads of the bolt. For purposes of this disclosure, xe2x80x9cboltxe2x80x9d will refer generally to any connection having external threads while xe2x80x9cnutxe2x80x9d will refer generally to any connection element having internal mating threads. When a bolt is passed through a medium and a nut is tightened over the bolt, it places the bolt in tension and the medium in compression. The medium can comprise any material to be fastened by the nut and bolt combination, such as engine parts, structural elements, pressure vessel flanges etc. In many instances, the amount of tension required in the bolt is predetermined and necessary to obtain a certain amount of compression in the medium.
Previous methods of tensioning include measuring the torque applied to the nut and estimating from that measurement the amount of tension on the bolt. This method, however suffers from the drawback that friction between the threads of the nut and the bolt may vary from one nut/bolt combination to next and is affected by a number of factors including coatings, corrosion, heat, dust, moisture, and manufacturing differences or imperfections.
Another method for accurately tensioning a bolt includes the use of a direct tension indicator, such as are discussed in commonly-assigned U.S. Pat. No. 5,769,581, issued Jun. 23, 1998, U.S. Pat. No. 5,931,618 issued Aug. 2, 1999, and U.S. Pat. No. 6,152,665 issued Nov. 28, 2000, all of which are issued to Wallace et al. and are wholly incorporated herein by reference. Reference is also made to U.S. patent application Ser. No. 09/613,993 filed Jul. 11, 2000 by W John A. Herr et al., said application also being incorporated herein by reference. In such devices, a washer-like element having protrusions is placed between the nut and the bolt head. The protrusions are calibrated to deform and flatten when the desired amount of bolt tension is reached. The amount of tension can then be determined based on the gap between adjacent elements caused by the direct tension indicator.
Direct tension indicators are currently available in the xe2x80x9cregularxe2x80x9d and xe2x80x9cself-indicatingxe2x80x9d types. The regular style requires the use of a feeler gauge to judge the residual gap closure. The self-indicating style include an elastomeric dye that becomes ejected from beneath the protrusions when the protrusions are flattened. Thus, with the self indicating style, the residual gap closure is judged by a visual assessment of the xe2x80x9csquirt event.xe2x80x9d
Both styles of direct tension indicators are used on bolts to control the tension of the bolt as it is being tightened.
It has been found, however, that friction between the threads of the nut and bolt can adversely affect the performance of these devices as well. Specifically, friction can undermine confidence in direct tensioning devices when the necessary effort to reach the specified bolt tension is much greater than expected due to the friction. Thus, when the torque resistance within the bolt assembly builds up to the point where the bolt tightening equipment cannot overcome it, the direct tension indicator protrusions are unacceptably high, and it may be declared that the bolt has not been tensioned correctly. Because of excessive torque buildup, therefore, the bolt tightening equipment is inadequate. The bolt installers often incorrectly attribute this failure to correctly compress the direct tension indicator protrusions to the direct tension indicator having been manufactured with protrusions that are too strong. This incorrect conclusion results in improper bolt tightening and controversial corrective procedures and extra cost.
It would therefore be desirable to provide a means for controlling the friction between the threads of the bolt and the threads of the nut to reduce the effort required to overcome friction effects and therefore improve user confidence in the direct tension indicators.
The above and other disadvantages of the prior art are overcome or alleviated by a friction reducing device comprising a stretchable thin film of elastomeric material impregnated or dusted with lubricating material. The thin film adheres to an annular member through which a fastener element having external threads can pass.